Such products require to comply with standards of ever-increasing severity, particularly with respect to flexibility (to reduce transmission of vibrations), their temperature behavior (inside and outside), their resistance to external agents, and where pipes are concerned, their impermeability relative to the fluid being conveyed. In addition, their lifetime must be as long as possible and their qualities of sealing and of impermeability must be maintained throughout said lifetime.
Proposals have already been made, in particular in patent application EP A-0732 384 in the name of the Applicant, to make thermoplastic-elastomer composite products in which adhesion between the thermoplastic and the elastomer is total because of modifying agents incorporated in the thermoplastic and in the elastomer, with the thermoplastic in said products acting as a strength member, as reinforcement, or as an insert, thereby making it possible, in elastomer section members in particular, to replace metal reinforcement by thermoplastic reinforcement for the purposes of reducing weight and of improving performance, while also simplifying manufacture.
Proposals have also been made on numerous occasions to make thermoplastic-elastomer composite pipes for conveying fluids in the automobile industry, which pipes generally comprise an inside layer of polyamide, an elastomer layer covering the polyamide, textile reinforcement, and an outer protective layer of rubber or of elastomer. In those known pipes, it is often necessary to make a compromise between the desired flexibility and impermeability to the fluid conveyed, and it is also necessary to interpose a thin layer of an appropriate adhesive material between the thermoplastic and the elastomer to ensure that they adhere to each other.
In the case of pipes for conveying coolant in an air conditioning circuit, it is necessary to improve the flexibility of the inside layer of polyamide while conserving its impermeability to the fluid conveyed (the CFC-containing coolants that have been used in the past are being replaced by HFCs known under the name R134a, in association with a lubricant of the polyalkylene glycol type), and proposals have been made, for example, for said inside layer to comprise a mixture of polyamide, a polyolefin modified by grafting, and of .epsilon.-caprolactam, however that composition does not adhere to the elastomer layer covering it. In addition, the quantity of .epsilon.-caprolactam used is a compromise between flexibility and impermeability to the fluid, with extra .epsilon.-caprolactam improving flexibility to the detriment of impermeability.